big o notation – Time complexity about Maximum subarray

I recently came across a function called the strawman algorithm which the pseudo code looks like this:

  Initialize bestsum = A(0)  
     For left=0 to n-1:
        For right = left to n-1:
           Compute sum A(left) + . . . + A(right)
           If sum > bestsum: bestsum = sum

The time complexity is Θ (n^3), and I don’t quite get where is the 3rd n comes from to get Θ (n^3)?